Magnetic resonance imaging is a well known and widely used method for obtaining medical images for both diagnostic and research purposes. In order to conduct a typical MRI procedure a volume of tissue is first placed in a static magnetic field. The tissue is then irradiated with radio frequency energy to tilt the nuclear magnetic moments within the tissue. Spin echo signals, which are characteristic of the irradiated tissue, are then recorded from the tilted nuclear magnetic moments. By using imaging techniques well known in the art, the signal contributions of individual volume elements (voxels) in the tissue are distinguished from each other. These voxels are ultimately displayed on a computer monitor or film for use by the physician or researcher.
For any system which relies on nuclear magnetic resonance techniques for acquiring data, one very important design consideration is the system's static magnetic field. For purposes of discussing the static magnetic field, consider an orthogonal x-y-z coordinate system. With the origin of this coordinate system at any point in the magnetic field, the magnetic field at that particular point can be characterized by the respective x, y and z components of the field and by spatial derivatives of the field strength. Specifically, the x, y and z components of the field magnetic vector, B.sub.0, are designated B.sub.x, B.sub.y and B.sub.z. The magnetic field can then be further characterized by the gradients which are the rate of change (first derivatives) of the field strength in the x, y and z directions. The field gradients are designated G.sub.x, G.sub.y and G.sub.z. It is to be appreciated that higher order derivatives may, and most likely are, present. For purposes of discussing the present invention, however, only the field gradients G.sub.x, G.sub.y and G.sub.z need be considered.
In a very general sense, a homogeneous magnetic field exists in a small neighborhood of a point where all of the field gradients, i.e. G.sub.x, G.sub.y and G.sub.z, are zero.
To by-pass the difficulties encountered with the manufacture of homogeneous MRI systems, recent efforts have been made to effectively use the more commonplace and more cost effectively established nonhomogeneous magnetic field. For example, U.S. Pat. No. 5,304,930 which is assigned to the assignee of the present invention, and which issued to Crowley et al. for an invention entitled "Remotely Positioned MRl System" (hereinafter the '930 patent) discloses a device and method for magnetic resonance imaging with a nonhomogeneous magnetic field. As clearly disclosed in the '930 patent, a nonhomogeneous field is a field that has a non-zero gradient G.sub.z. Regardless whether the magnetic field is homogeneous or nonhomogeneous, in order to perform an MRI procedure it is necessary to distinguish various voxels within the tissue to be imaged. To do this, the tissue is typically encoded with spatial patterns.
One widely recognized encoding procedure for imparting spatial patterns in the tissue volume to be imaged involves the application of gradient magnetic fields. These so-called gradient magnetic fields consist of an additional range of field values, denoted by .DELTA. B.sub.o, that are superimposed on the static field. At this point it should be noted that the x and y spatial variations of .DELTA. B.sub.o are determined by the respective x and y gradients, G.sub.x and G.sub.y, of the superimposed field values. Through the Larmor constant, a range of Larmor frequencies determined by the expression EQU .DELTA.f=.lambda..DELTA.B.sub.o
exists during the application of the gradient, either G.sub.x or G.sub.y or both. The effect of this range of Larmor frequencies is a spatial pattern of phase accumulation in the magnetic moments across the tissue of interest. For the purposes of the present invention, the key point is that the range of Larmor frequencies associated with a gradient field (G.sub.x or G.sub.y) spatially distinguish one voxel from another in the respective x and y directions. With a suitable number of gradient encodings, which are each followed by a measurement of spin echo signals, data is obtained that can be reconstructed into an image of the array of voxels.
Due to the fact the present invention contemplates an MRI operation with a z-gradient (G.sub.z), several consequences which involve data acquisition and the suppression of exogenous noise are pertinent. First, the data acquisition techniques in the presence of a z-gradient are quite different from those used for conventional MR1 in a homogeneous magnetic field. This data acquisition aspect has been fully considered and disclosed in U.S. Pat. No. 5,304,930, which has been cited above and which is incorporated herein by reference. Second, the suppression of exogenous noise is accomplished by imposing a z-gradient, G.sub.z, which is greater than either the x or y encoding gradients (G.sub.x and G.sub.y). An additional benefit from this relationship between the gradients is the fact that the system is less sensitive to static field perturbations.
Spread spectrum techniques are widely used in the communications industry to avoid the corruption of transmitted signals by interfering noise sources. The method is particularly effective in the presence of a discrete set of noise sources that occupy narrow frequency bands. Conventional radio or television signals fall into this category.
The basic idea in spread spectrum techniques is to send and receive signals that occupy a range of frequencies that is significantly wider than that of the individual interfering noise sources. In this manner, the effects of the individual noise sources are minimized.
The use of such techniques is not conventionally taught in the art since, as mentioned above, most magnetic resonance equipment can be shielded from the effect of external noise by enclosing the system in an r.f. shielded room (Faraday cage). However, the present invention recognizes there are benefits to systems that are not enclosed in shielded rooms, especially for cost of operation and smaller portable systems.
In light of the above, it is an object of the present invention to provide methods for acquiring data from voxeis in a contour of tissue which is accomplished using a z-gradient, G.sub.z. It is another object of the present invention to provide methods for acquiring data from voxels in a contour of tissue which uses an extensive z-gradient, G.sub.z, to suppress exogenous noise making the data less sensitive to static field perturbations. Still another object of the present invention is to provide methods for MRI which are relatively easy to accomplish and comparatively cost effective.